Most hemipterans feed on plants, using their sucking and piercing mouthparts to extract plant sap. Some are parasitic while others are predators that feed on other insects or small invertebrates. They live in a wide variety of habitats, generally terrestrial, though some species are adapted to life in or on the surface of fresh water. Hemipterans are hemimetabolous, with young nymphs that somewhat resemble adults. Many aphids are capable of parthenogenesis, producing young from unfertilised eggs; this helps them to reproduce extremely rapidly in favourable conditions.

Humans have interacted with the Hemiptera for millennia. Some species are important agricultural pests, damaging crops by the direct action of sucking sap, but also harming them indirectly by being the vectors of serious viral diseases. Other species have been used for biological control of insect pests. Hemipterans have been cultivated for the extraction of dyestuffs cochineal (also known as carmine) and for shellac. The bed bug is a persistent parasite of humans. Cicadas have been used as food, and have appeared in literature from the Iliad in Ancient Greece.